The IEEE 802.11 standards are used internationally to set the characteristics of wireless local area networks or WLANs and allow interoperability of all the computing devices wirelessly connecting to them. Also often designated as “Wi-Fi networks” this latter appellation makes reference to the certification given by the “Wi-Fi Alliance”, a trade association aimed at certifying interoperability between devices adhering to the 802.11 standards. The label “Wi-Fi” is delivered to materials that meet the corresponding specifications. A Wi-Fi network is thus not different of a network based on the 802.11 standards.
Thanks to 802.11 standards, it has been possible to create wireless LANs offering broadband connections. This type of networks has become very popular in open areas hosting potentially a high concentration of users like stations, airports, hotels, trains, etc. Users are thus given the opportunity to establish a wireless network with a broadband connection to the Internet within an area, typically covering several tens of meters indoors, in the vicinity of an access point (AP) or “hotspot”. With the evolution of the standards, the speed of the connection a user can establish has increased, in practice, from a few megabits per second (Mb/s) with the early version of the standards up to hundreds of Mb/s with 802.11n, a version which was officially released in late 2009.
To allow 802.11 broadband connections to reach the highest speed values mentioned above, 802.11n version of the standards has introduced the possibility of wider communication channels. While initially being of 20 MHz the channel width can optionally be doubled thus brought to 40 MHz. A more recent evolution of the standards, namely the 802.11ac version, has extended this possibility to channel widths of 80 and 160 MHz.
Then, in an 802.11 compliant WLAN, this poses the problem of interoperability of the various communications devices forming a wireless network, i.e., a basic service set (BSS) comprised of all the stations (STAs), such as laptop computers or smart phones, participating to the wireless network at any point of time and of an access point (AP) acting as a master to control the stations within the BSS. More specifically, all communications devices must then be able to detect on the fly, for each data frame received, which type of bandwidth is actually used.
Thus, the invention is generally aimed at assessing on the fly the bandwidth of each received frame in an environment where 802.11n has introduced optional 40 MHz channel width operation of basic service set or BSS complying with this version of the standards. However, BSS must also maintain interoperability with older or legacy stations and with high throughput (HT) stations that operate only with 20 MHz channel. Such BSS must be capable of accommodating three basic classes of device: 20 MHz legacy or non-HT stations, 20 MHz HT stations, and 20/40 MHz HT stations.
Hence, in this latter case, in an 802.11n 40 MHz BSS, a 20/40 MHz STA or AP has to be capable of transmitting and receiving, indifferently, 40 MHz frames in a 40 MHz channel, and 20 MHz frames in a 20 MHz primary channel defined within the 40 MHz one.
FIGS. 1a to 1c illustrate the frequency location of the 20 MHz primary channel in a 40 MHz BSS compared to the 40 MHz channel 100. The 20 MHz primary channel 120 can occupy either the lower 114 or the upper band 116 of the 40 MHz band 110 depending on the BSS configuration. The other defines a secondary channel 130.
Although the central frequency of the 20 and 40 MHz channels are then different and because it would not practically feasible nor convenient to adapt the characteristics of the radiofrequency (RF) components, i.e.: RF synthesizer frequency and cut-off frequency of the analog filter, to each transmit or receive frame, a 20/40 MHz capable STA keeps maintaining its RF carrier frequency to the central frequency 112 of the 40 MHz frames. This is particularly relevant in reception mode since a 20/40 MHz capable STA has no way of predicting if the bandwidth of the next incoming frame is going to be 40 or 20 MHz.
FIGS. 2a to 2c illustrate the cases corresponding to the version 802.11ac of the standards where 80 MHz capable BSS can be formed. Then, in an 802.11 ac 80 MHz BSS, a 20/40/80 MHz capable STA or AP must be able to transmit and receive indifferently 80, 40 and 20 MHz frames, respectively, in the 80 MHz main channel 200, in the 40 MHz primary channel 210 and in the 20 MHz primary channel 220. FIGS. 2a to 2c are just an example of location of the different channels. Primary and secondary locations can be reserved, but the 20 MHz primary channel must be within the 40 MHz primary channel. This version of the standards also allows the aggregation of two 80 MHz channels to obtain a 160 MHz channel (not shown).
To perform the reception of the above various frame types the modem receive path of a STA or AP capable of operating in a multi-channel-widths environment must be configured as a function of the bandwidth of the frames to be demodulated. For example, the demodulation of a 20 MHz frame requires settling computational resources to perform a 64-point Fast Fourier Transform (FFT), whereas the demodulation of an 80 MHz frame requires that a 256-point FFT be performed.
However, as already discussed, in the case of 802.11n and later versions of the standard, modem receivers do not know in advance the actual bandwidth of a next incoming frame. Although this information is indeed contained, as shown in FIG. 3, in the so-called HT-SIG (or VHT-SIGA with 802.11 ac version of the protocol) fields 340 that precede the data symbols 350 of each received frame, depending on the architecture and implementation of the modem, this information may in practice come too late to be readily usable.
Hence, having to wait that SIG fields 340 be demodulated to configure the modem in accordance with the current bandwidth of the received frame, in order to demodulate properly the following data symbols, is not actually a viable solution. The standard frame format is the so-called mixed format (MF) 320 shown in FIG. 3. A so-called, shorter, Green-field (GF) frame format 310 is also used for which the above problem is even more stringent since this type of frame contains the HT-LTF field (High Throughput Long Training Field) before the HT-SIG (High Throughput SIG field). Also, one must ensure good backward compatibility with the 802.11 legacy frames 330 corresponding to the previous versions of the standards, namely the 802.11a to g versions.
Thus, it is a particular object of the invention to disclose a method to assess the bandwidth of an incoming frame on the basis of its preamble section 330, i.e., prior to the decoding of the above SIG fields.
Further objects, features and advantages of the present invention will become apparent to the ones skilled in the art upon examination of the following description in reference to the accompanying drawings. It is intended that any additional advantages be incorporated herein.